-Noise is an externality of transport that may affect our environment and our health. It is important to focus on the noise emission of electric vehicles and their potential for noise reduction. I will start with some basic definitions for noise. What is noise? It is an acoustic phenomenon producing an unpleasant or disturbing auditory sensation. This definition is based on a subjective and negative notion. But underlying this subjective notion, there is a physical phenomenon, a pressure wave that moves through the atmosphere. Sound pressure is indeed defined as the local pressure deviation from the ambient atmospheric pressure. The principal quantities that characterize sound are: the level, or amplitude, or intensity, or volume, expressed in decibels, which corresponds to the logarithmic transformation of the acoustic pressure, and the frequency, which determines if a sound is high, low, or medium. Frequency is expressed in Hertz. The average human ear is sensitive to sounds with frequencies between 20 and 20 000 Hertz. Sound level is sometimes expressed in A-weighted decibels, or dB(A). This corresponds to a weighting of the sound level that takes into account the sensitivity of the ear depending on the frequency. This decibel scale provides us with indicators ranging between 0 dB, the threshold of audibility, and 120 dB, the threshold of pain. Between the two are characterized sound levels of calm situations, and of annoying, harmful, or even hazardous noises. Transport noise levels usually range from 60 to 90 decibels, which makes them potentially harmful to our health. The decibel logarithmic scale leads to peculiar arithmetics when sources add up. When two sound sources emit simultaneously at the same level, the resulting level is 3 decibels higher, rather than twice the level of each. Also, if a source is divided by two, for example road traffic, the resulting sound level is only 3 dB lower. Another consequence is the masking effect. When two sources emit simultaneously at very different levels, the louder one predominates, provided it is at least 10 dB louder than the other. This is very important, because in order to reduce noise in a given situation, one must target the loudest source. I will now present the sources of noise of a vehicle. Consider thermal vehicles. One usually distinguishes mechanical noise sources, i.e. the engine, exhaust, and transmission, from sources of tire/road or rolling noise. The latter predominates for light vehicles moving faster than 50 km/h, which corresponds to third gear ratio, and for heavy goods vehicles moving faster than 80 km/h. The relative importance of these noise sources depends the vehicle type, passenger car, heavy goods vehicle, or two-wheeler, and on its kinematics, i.e. its speed, acceleration, and engine rate. This graph, deliberately without values, shows the evolution of these sources as a function of speed. We can see the steady evolution of transmission and exhaust noises, the linear evolution of engine noise for each gear, at higher levels, and tire/road noise, which grows logarithmically with speed and rapidly becomes louder than all the other noise sources. For electric vehicles, it is important to note the absence of exhaust noise and the considerable reduction of engine noise, which causes tire/road noise to predominate at all speeds. Also note that recent regulations require electric vehicles to be fitted with a device that produces a warning sound at very low speed, under 20 or 30 km/h. This source adds to the electric vehicle's own noise. I will now illustrate the acoustic performance of electric vehicles with some experimental results. First, here are noise emission measurements of light vehicles of a same range, measured at a constant speed. The red curve shows the results for an electric vehicle, and the other colors show the results for thermal vehicles, diesel, petrol, and LPG. We see that in first gear ratio there is a difference of around 10 dB between the electric vehicle and the others, 12.5 dB precisely for the diesel vehicle. This difference drops to 4 dB in second gear ratio, and in higher gear ratios, this difference becomes negligible as rolling noise predominates over other noises and is roughly equivalent for all vehicles. The second example pertains to motor scooters. The green curve shows the results for an electric scooter at constant speed. We also see a difference of approximately 10 dB with the scooters with combustion engines. This difference is even greater during acceleration or deceleration, where the difference can reach up to 15 dB. In conclusion, the external noise generated by electric vehicles is mainly attributable to rolling noise, and significant improvement in sound level is achieved at speeds where thermal engine noise predominates. The improvement is therefore very significant at low speeds, it decreases as speed increases, and becomes negligible over approximately 50 km/h. On the scale of traffic, the benefits of electric engines are significant only if the proportion of electric vehicles is substantial. This simulation shows that for electric vehicles that emit 10 dB less noise than thermal vehicles, if electric vehicles make up 25% of the traffic, the overall noise reduction is just over 1 dB. If they make up 50% of the traffic, the overall noise reduction is approximately 2.6 dB. For electric vehicles that emit 4 dB less noise than thermal vehicles, at higher speeds, for example, the overall noise reduction with 25% electric vehicles is less than 1 dB, 0.7 precisely. And the overall noise reduction with 50% electric vehicles is approximately 1.6 dB. I will now compare the potential of electric vehicles with the other means of reducing road noise. Measures at the source involve improving vehicle components and tires, and electric vehicles fall within this scope. Other measures include traffic management, for example reducing traffic volume, by setting up detours, or by promoting public transportation, reducing traffic speed, improving traffic flow, and promoting soft modes of transport. Another type of measure consists in changing the road surface so as to reduce tire/road noise. Other types include measures on propagation and reception. They consist in building noise barriers, earth embankments, partial or total covers, that are very effective but can be very costly. Another measure is to act on the urban planning, by playing with the layout and geometry of thoroughfares, with the location of buildings or of their room distribution with respect to noisy thoroughfares. Finally, façade insulation can also be improved, particularly window insulation. In summary, there is no one ideal solution among all these means of reducing noise, but they are all complementary measures among which electric vehicles will find their place, and each measure must be adapted to the context.
